Ozone Designations

Ozone Designations Guidance and Data

The information on this page is intended to support the area designation process for the 2015 ozone National Ambient Air Quality Standards (NAAQS) by providing states and tribes with current data and tools that may be useful in evaluating each area on a case-by-case basis and in making boundary recommendations. The data and tools that could be of use for these evaluations are not limited to the data and tools provided here.

B. Summary of Five-Factor Analysis

The five factors identified in the Guidance for Area Designations for the 2015 Ozone NAAQS are listed below, along with data and data sources that may be useful in evaluating each area on a case-by-case and in making boundary recommendations. The following is not an exclusive list of factors, data, or sources of data that could be considered in assessing an area. EPA is providing this list as a useful tool for the designations process, and it should not be construed as representing a decision by EPA to rely solely on this list for final designation determinations. EPA intends, at a minimum, to evaluate these factors, data and/or data sources in making final determinations regarding area designations for the 2015 Ozone NAAQS. States and tribes should refer to the designations guidance for more complete information about these factors and the factor analysis. The EPA recognizes that consideration of the 5 factors for an assessment can be resource intensive. In addition to providing the relevant data to facilitate the analyses, the EPA is providing an Ozone Designations Mapping Tool to assist air agencies in developing their area designation and nonattainment boundary recommendations. The Ozone Designations Mapping Tool is available below.

Factor 1: Air Quality Data - The air quality analysis is an examination of available ambient ozone air quality monitoring data, including the annual design value calculated for each area based on air quality data for a 3-year period.

Factor 2: Emissions and Emissions-Related Data –The emissions analysis examines emissions of precursors (NOx and VOCs) that form ozone in the county with the violating monitor and in nearby counties. Emissions data are derived from the 2011 National Emissions Inventory (NEI) version 2, and are given in tons per year. The 2011 NEI version 2 data will be the most recent NEI information available at the beginning of the designations process. When the 2014 NEI becomes available, then it will be provided in lieu of the 2011 NEI version 2. Emissions data indicate the potential for a source to contribute to observed violations, making it useful in assessing boundaries of nonattainment areas.

Factor 3: Meteorology - The evaluation of meteorological data helps to determine the effect on the fate and transport of emissions contributing to ozone concentrations and to identify areas potentially contributing to the monitored violations. One basic meteorological analysis involves assessing potential source-receptor relationships in the area using summaries of emissions, wind speed, and wind direction data. A more sophisticated assessment involves modeling air parcel trajectories to help understand complex transport situations. The HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) modeling system may be useful for some areas to produce trajectories that illustrate the 3-dimensional paths traveled by air parcels to a violating monitor. The EPA is providing back trajectories in the Ozone Mapping Tool for violating monitors, for each day of high ozone concentration (i.e., daily maximum 8 hour values that exceed the NAAQS) at those monitors. States or tribes can choose to do additional HYSPLIT modeling and guidance is provided in the ozone designations guidance document.

Factor 4: Geography/Topography - The geography/topography analysis includes an examination of physical features of the land that might define the airshed and, therefore, affect the formation and distribution of ozone over an area. Mountains or other physical features may influence the fate and transport of emissions and ozone concentrations. Additional analyses may consider topographical features that cause local stagnation episodes via inversions. Valley-type topographical features can cause local stagnation episodes where vertical temperature inversions effectively “trap” air pollution. Under these conditions emissions can accumulate leading to periods of elevated ozone concentrations. These inversions may be limited in extent and, therefore, the areas with inversions may need to be separated from areas at altitudes above the top of the inversion layer in locations where exceedances are associated with this type of event.

Factor 5: Jurisdictional Boundaries - Existing jurisdictional boundaries may be considered for the purposes of providing a clearly defined legal boundary and carrying out the air quality planning and enforcement functions for nonattainment areas. Examples of jurisdictional boundaries include, but are not limited to: counties, air districts, areas of Indian country, metropolitan planning organizations, and existing nonattainment areas. If an existing jurisdictional boundary is used to help define the nonattainment area, it must encompass all of the area that has been identified as meeting the nonattainment definition. Where existing jurisdictional boundaries are not adequate to describe the nonattainment area, other clearly defined and permanent landmarks or geographic coordinates should be used.

C. Ozone Designations Mapping Tool

The ozone designations mapping tool gives you access to air quality data, emissions data, and jurisdictional boundaries to assist in designations for the 2015 Ozone NAAQS